During design, production, or maintenance, an aircraft turbojet is tested in a specific test bench. Such a test bench makes it possible to test the real performance of the turbojet, while taking measurements therein. The flow rates, the temperatures and the pressures can be measured there in particular during real performance test phases. Now, during these real tests, the turbojet emits sound waves that can exceed 140 dB. Conventional passive exhaust silencers are inadequate in the low frequencies where the input impedance of an absorbent coating would be so high as to virtually prevent the acoustic flow through the coating. These sound waves also excite the items of equipment of the test bench into vibration and can damage them.
Furthermore, these waves propagate into the control rooms and outside the test bench, where they harm the environment. The noise level significantly disturbs the occupants of neighbouring or distant homes. The zone affected by the acoustic power of the turbojet is particularly vast, especially in the case of large aircraft turbojets, or even turbojets operating with an afterburner.
While the active cancellation technologies have been developed for different environments, the conventional approach consisting of generating a cancellation wave cannot easily be tailored to the problem of the exhaust from a jet engine or to any other application where the speed of flow in the passage is high enough to interfere with the detection of the approaching sound wave. Moreover, the properties of the exhaust jet are hostile because of their temperature, their turbulence, and the presence of corrosive gasses.
Consequently, a direct detection and control interface for the exhaust conduit is not easy to implement. Furthermore, the previously known active acoustic attenuation systems achieve noise attenuation by introducing, into the channelling, a cancellation sound that is ideally a mirror image of the incoming undesirable noise. This cancels the noise downstream of the noise source and the introduction of a new sound wave that propagates in the upstream direction. However, if the source of the introduced noise does not absorb the upstream noise, since there is no effective dissipating section between the noise source and the transducer, the acoustic energy can form at a high level in the upstream conduit. This results in high acoustic pressure through the active silencer section, but only a much smaller reduction of the noise propagating in the downstream direction.
In the field of active silencers, it is known to place loudspeakers in a chamber accommodating a conduit for collecting the exhaust jet. The loudspeakers surround the conduit, and are actuated to reduce the acoustic pressure there. The acoustic flow thus increases through the porous wall, which in turn increases the dissipation of the acoustic energy in the conduit.
The document WO 95/19075 A2 discloses a turbojet test bench. The test bench has an exhaust with an active silencer acting on the low frequencies. The silencer processes frequencies from 10 to 80 Hz for a turbojet. It includes a passage surrounded by a porous layer, and several annular chambers around the porous layer. Several microphones are connected to a controller that controls loudspeakers placed in the annular chambers. This system effectively makes it possible to reduce the noise level emitted out of the test bench. This device is particularly efficient in the low frequencies. However, its efficiency remains limited.
The patent document U.S. Pat. No. 5,662,136 A discloses another test bench for turbojet.